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Magic-sized clusters (MSCs) are kinetically stable, atomically precise intermediates along the quantum dot (QD) reaction potential energy surface. Literature precedent establishes two classes of cadmium selenide MSCs with QD-like inorganic cores: one class is proposed to be cation-rich with a zincblende crystal structure, while the other is proposed to be stoichiometric with a “wurtzite-like” core. However, the wide range of synthetic protocols used to access MSCs has made direct comparisons of their structure and surface chemistry difficult. Furthermore, the physical and chemical relationships between MSC polymorphs are yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs are contrasted using a combination of 1H NMR spectroscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV–vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV–vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder XRD and PDF analysis. This work helps to simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure–property relationships in colloidal semiconductors through atomically precise synthesis. 

Abstract With rising CO₂emissions and growing interests towards CO₂valorization, electrochemical CO₂reduction (eCO₂R) has emerged as a promising prospect for carbon recycling and chemical energy storage. Yet, product selectivity and electrocatalyst longevity persist as obstacles to the broad implementation of eCO₂R. A possible solution to ameliorate this challenge is to pulse the applied potential. However, it is currently unclear whether and how the trends and lessons obtained from the more conventional constant potential eCO₂R translate to pulsed potential eCO₂R. In this work, we report that the relationship between electrolyte concentration/composition and product distribution for pulsed potential eCO₂R is different from constant potential eCO₂R. In the case of constant potential eCO₂R, increasing KHCO₃concentration favors the formation of H₂and CH₄. In contrast, for pulsed potential eCO₂R, H₂formation is suppressed due to the periodic desorption of surface protons, while CH₄is still favored. In the case of KCl, increasing the concentration during constant potential eCO₂R does not affect product distribution, mainly producing H₂and CO. However, increasing KCl concentration during pulsed potential eCO₂R persistently suppresses H₂formation and greatly favors C₂products, reaching 71 % Faradaic efficiency. Collectively, these results provide new mechanistic insights into the pulsed eCO₂R mechanism within the context of proton‐donator ability and ionic conductivity.